Because of the increasing concern with environmental pollution it is a common practice to scrub waste or flue gases before they are to be discharged into the atmosphere in order to remove toxic, noxious and corrosive substances therefor. For example, in the case of combustion exhaust from oil and coal fired furnaces, e.g. in steam and other heating plants or in power plants, the gases to be discharged into the atmosphere and arising at the outlet of the combustion chamber may contain greater or lesser quantities of sulfur dioxide, depending upon the sulfur content of the fossil fuel which is used. Sulfur dioxide is a highly corrosive, toxic and noxious material which cannot be released into the atmosphere at the concentrations at which it may be present in many combustion gases under applicable environmental health and safety regulations.
It contributes to acid rain and endangers the natural environment especially forests and lakes, and when precipitated with rain in occupied areas, causes damage to paint, metal objects and the like while detrimentally affecting the health of the residents and animal life in the affected area.
It is thus known and required, in many instances, to provide scrubbers capable of washing sulfur dioxide inter alia from such combustion gas streams before they are discharged into the atmosphere. The scrubbers generally intensively mix water in finely divided form with the gas so that the scrubbing liquid acts in part as a mechanical trap, in part as a solvent and in part as an adsorbent for substances to be removed from the gas stream including the sulfur dioxide.
It is known, in this connection, to increase the scrubbing effectiveness of any scrubbing column capable of bringing about an intensive mixing of the gas and the liquid and an intimate contact therebetween, by promoting the chemisorptive effect, e.g. by addition of lime to the scrubbing liquid.
The lime (CaO) promotes removal of SO.sub.2 from the gas phase by the formation of CaSO.sub.3 therewith.
It is desirable, from a point of view of removing this material as a low solubility solid from the liquid, to form the calcium sulfate CaSO.sub.4 by oxidation. However, when such oxidation occurs the calcium sulfate precipitates out almost immediately and encrustation of the ducts and passages therewith poses a significant problem.
In practice because the SO.sub.2 concentration in the gas fluctuates, it is difficult to contrive the pH and calcium oxide content of scrubbing liquid and its quantity so that encrustation is avoided. Hence systems have been described in literature to overcome this disadvantage by incorporating in the wash liquid, oxidation-retarding agents. These oxidation-retarding agents are not only expensive but are inconvenient to use and hence involve other disadvantages.